Fix ERC20 token approval

[andrevmatos]

Fixes #2010
Fixes UDC deposit part of #1908

Short description
This makes 2 important changes:

• To approve before depositing (both for depositChannel and depositToUDC), if the current allowance is less than required to deposit but different than 0, we must first set it to 0 then set the needed allowance; this is a requirement on secure ERC20 contracts like RDN
• To avoid the double transactions as above, we now default to approving MaxUint256 (i.e. a very big number), trusting the spender (i.e. TokenNetwork contract and UserDeposit contract, respectively); this way, only an approve per token is needed, and the following deposits can happen without the need to mine approve again
  • If a user isn't confortable with the above, they can either set config.minimumAllowance to a small value, like Zero, which will make it default to the strictly requested deposit amount, and/or, on Metamask's prompt (per approve transaction), simply set the allowance to any desired amount (as long as it's greater than or equal the requested deposit, or else the deposit tx will fail)

Additionally, this also refactor the depositToUDC logic, which was on the public Raiden class, to its own epic, and interact with it through udcDeposit async actions.

Definition of Done

• Steps to manually test the change have been documented
• Acceptance criteria are met
• Change has been manually tested by the reviewer (dApp)

Steps to manually test the change (dApp)

1. Test the default requested approve is now a very big number
2. Test editing allowance on Metamask and setting it to a value greater than the requested deposit will succeed (don't forget to also increase a little the gasLimit), but on the next deposit, it'll be required to first approve(0) and then the new value, which still defaults to a very big number

[codecov]

Codecov Report

Merging #2018 into master will decrease coverage by 0.01%.
The diff coverage is 97.18%.

@@            Coverage Diff             @@
##           master    #2018      +/-   ##
==========================================
- Coverage   95.96%   95.94%   -0.02%     
==========================================
  Files         155      155              
  Lines        5896     5922      +26     
  Branches     1057     1121      +64     
==========================================
+ Hits         5658     5682      +24     
- Misses        179      181       +2     
  Partials       59       59              

Flag	Coverage Δ
#dapp	93.19% <ø> (ø)
#sdk	97.07% <97.18%> (-0.03%)	⬇️
#sdk_e2e	65.82% <95.77%> (+0.06%)	⬆️
#sdk_unit	86.24% <74.64%> (+0.20%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
raiden-ts/src/channels/utils.ts	96.77% <86.66%> (-2.03%)	⬇️
raiden-ts/src/channels/epics.ts	97.87% <100.00%> (-0.03%)	⬇️
raiden-ts/src/config.ts	100.00% <100.00%> (ø)
raiden-ts/src/epics.ts	96.49% <100.00%> (ø)
raiden-ts/src/helpers.ts	94.92% <100.00%> (+0.15%)	⬆️
raiden-ts/src/raiden.ts	91.46% <100.00%> (-0.19%)	⬇️
raiden-ts/src/services/actions.ts	100.00% <100.00%> (ø)
raiden-ts/src/services/epics.ts	99.70% <100.00%> (+0.02%)	⬆️
raiden-ts/src/utils/actions.ts	96.34% <100.00%> (ø)
raiden-ts/src/utils/types.ts	95.89% <100.00%> (+0.11%)	⬆️

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[weilbith]

I haven't looked into much details of the tests. Sorry.
Rest in the comments. Function wise this should be fine. Therefore you get the approval (since this seems to be the convention).

[weilbith]

Thanks for your adjustment! 🤗