Add support for Mastodon and Pixelfed profile links

[waschinski]

Discussed in #80

Similar to how Twitter and Instagram are already implemented but probably with a complete URL to the profile as an env setting.

[SebStange]

Due to recent changes in the social network sphere, a more general approach to add external links might be more future proof (BlueSky, GitLab, Photo Communities...).

[waschinski]

I am currently playing with an alternative configuration option which is using a JSON string and Simple Icons. Set up would most likely look like so:

AUTHOR_LINKS='["github":"https://github.com/maxvoltar","x":"https://twitter.com/maxvoltar","instagram":"https://www.instagram.com/maxvoltar"]'