move tpa issues into subprojects or cleanup labels

we want to move tpa issues into subprojects instead of using labels for sorting tickets.

argument for labels:

• projects are for code
• labels are more lightweight
• all issues are in the same project, make it clear for newcomers where to file issues

arguments for projects:

• labels are for things that can have multiple values, which is rarely the case for components
• labels are kind of a mess right now (see #3 (closed) and #4 (closed)) so it is hard to figure out what is what
• in particular, it's really hard for users to find components when filing an issue. try, for example, to see which components exist in the "Label" dialog in https://gitlab.torproject.org/tpo/tpa/services/-/issues/new?issue%5Bassignee_id%5D=&issue%5Bmilestone_id%5D= (assuming they are not prefixed with component:: because that will presumably change in #4 (closed) and anyways it's too long so it gets trimmed everywhere)
• on GitLab, projects can be "just issues" without a repository or wiki (as opposed to GitHub)