Add deprecation warning for network change page reload

[danfinlay]

Need to communicate somehow (maybe a tweet or small blog post) that we will change reload behavior on network change.

[danfinlay]

Actually, before we communicate this, let's provide a migration path. How about an event devs can subscribe to for network change?

[danfinlay]

Add a guide to detecting network change.

[danfinlay]

I started this blog post, and realized that if we deprecate this before we support subscriptions, it will mean asking devs to use polling, which is awful.

Instead, I am marking this issue blocked by web3 1.0 support, so we should first:

• merge Web3 1.0 subscription support #4047
• Complete either Add Web3 1.0 "subscription" support with a polling subprovider #3642 or Add Web3 1.0 "subscription" support with a polling subprovider #3642.

I started a draft on Medium that I can resume when those are done.

[bdresser]

article published here: https://medium.com/metamask/breaking-change-no-longer-reloading-pages-on-network-change-4a3e1fd2f5e7