Add donation link

[viktorix]

We need to help support developers. If they want to, they should be able to add a donation link. It will be shown:

• profile page
• individual plugin page
• individual theme page

I've already added text input to the profile page (97aa8ab).

For reference, this is what WordPress shows:

TODO

• Add donation link to individual pages
• Add donation link to dev profile page

[viktorix]

Added donation box to individual pages. Now need to add something to the dev profile page.