Add option to disable fetching pronouns from external sites

[aurorasmiles]

Some users might not want their client to fetch pronouns from external pages (like pronouns.page or pronouns.cc) or from external instances. It would be nice to have a "privacy mode" option, where pronouns are only retrieved from profiles cached on your home instance/the instance you are currently browsing on.
This should be disabled by default, since I assume it severely reduces ProToots' functionality, but it still might be something nice to have that I'm pretty sure some people would appreciate

[nachtjasmin]

Needs to be done anyway with #15, because the permissions for those requests are optional by default and pronouns.page support needs to be explicitly enabled then.

All other pronoun pages, e.g. pronouns.cc just work by extracting the pronouns from the URL, no external requests are done for this.