[ux] Staking tab warning

[jzbz]

As discussed on Matrix: https://matrix.to/#/!DwNysKhaQPBPURaBGt:decred.org/$d4D_hx7x0rSWNdhBpwK3RfH6hP8zb4jyjL3Hm7n-LD8?via=decred.org&via=zettaport.com&via=t2bot.io

Idea would be to show a few slides when the staking tab is clicked explaining the basics of tickets/staking and warning users about funds being locked.
Could look similar to the way the LN tab warning works.

Text can be derived from whatever the final copy of #3669 ends up being.