Bugfix/3922 no duplicate requests

related to this: https://labs.riseup.net/code/issues/3922

Originally was a workflow bug:

* blue invites red to join animals
* red requests membership in animals
-> two requests pending.

after one request has been accepted the other one will still be pending etc.

Turns out this is no regression but has been that way. So I made some general fixes to html_safety and translations in requests and made it so the link to create a request to join after having been invited is replaced with a "you have been invited + link".

In order to really fix the underlying problem i think we should touch the models and it's beyond the scope of this rework. Created an issue for it.