Add donation popup

[Popolechien]

Signal has a donation prompt that appears every three months or so at the bottom of its android app. Nothing fancy (see text below - it's in French but the gist of it is Donate today / Signal needs your help with the choice button being remind me later of donate)

There is already a static feature in the settings, so this is in addition to the latter.

Signal has a sophisticated donation funnel that allows in-app payments, but at this stage a link to support.kiwix.org would already be good

[kelson42]

@MohitMaliDeveloper Please implement something like in Signal.

[kelson42]

The donation does not look well on tablet in dark mode, please make abetter proposal.