Add config for periodic donation banner

[virgile-dev]

Is your feature request related to a problem? Please describe. Most people don't know they can contribute by donating money to uMap maintainers. The info is only available on umap-project.org and Github.

Describe the solution you'd like Thunderbird's periodic full screen donation banner has shown outstanding results. See their FOSDEM talk about it. They also share their choices in term of donation stack. I'd like for uMap to try and implement a similar banner. It could be a configuration option that can be enabled or disabled by the instance admin. I guess it would be interesting to be able to define the periodicity by which it would be displayed to users. Of course we'd have to determine where we'd display it, I'm guessing not on iframes. Also we'd need great design (ping @Aurelie-Jallut) and copy (ping @C-Sophie) writing and translation in many languages.

Looking forward to see the discussion in the comment section to see if we can figure this out.