Add UI to assign tags

[ChristophWurst]

This is another split-off from #4805 for the UI part of assigning tags to a message.

As of #4665 we have full back-end support for user tags and each user gets five default tags assign by default. However as a user you can currently only see and influence the important tag. We need a UI mechanism that allows you to assign any other configured tag to a message.

If I remember we had some possible ideas for this

• The envelope/message actions menu gets an entry *Tag as ... and when you click the action is replaced with n actions that each represent a certain tag
• The envelope/message actions menu get an entry Tag as ... and when you click a sub action menu expands, showing an action for each configured tag
• There is a separate actions menu just for tags and it only lists the available tags for the user to click. I don't recall where we wanted to put the trigger for the menu, but possibly that was meant for the envelope/message component. @miaulalala wasn't that your idea?

#4806 is required for user-defined tags, but that doesn't block this ticket.

• Requires a restructured Vuex store that works with tag mutations Normalize envelope tags in vuex store #4902

cc @nimisha-vijay :)