Modmatrix: find a maintainable alternative for the possible headers + icons.

[creesch]

Currently modmatrix fetches the possible actions from the dropdown in the menu which also makes it possible to grab the icons from the current reddit css. This will likely not be possible in the future.

Ideally we get the admins to include something in the front-end api but if that is not possible we have to put the list of possible actions somewhere in the code where we can easily maintain it and make the code robust enough to handle actions it doesn't know.

[creesch]

This issue was moved to toolbox-team/reddit-moderator-toolbox#7