Add ability to crowdfunding donations for proposals authors

[MurzNN]

At now many users provide interesting proposals to https://matrix.org/docs/spec/proposals list, and sometimes other people want to speed-up developing of some proposal, needed for him. At now he can only give "+1" reaction on github or spam in comments. Will be good to add ability for give donations directly to needed proposal, that will get his author after implementing it.

[MurzNN]

Here https://www.documentfoundation.org/foundation/grant-request/ is example of same feature in Libreoffice proposals, and bountysource.com is good service for implement this.

[Half-Shot]

Bounty source could well work for this. People have done so in the past for issues, so why should proposals be done differently.

Obviously it's a bit trickier with what counts as a complete proposal given how much they evolve over time, and that may not reflect the donors wishes. Regardless, a good idea