Use url_name in urls

[Baschdl]

Use the url_name from the configuration as p_type in the urls etc.

"A" and "B" are still in the dictionaries for programmer's convenience

Closes #113

[Baschdl]

@feeds Can we use instead of https://github.com/match4everyone/match4everything/blob/2341e57c775cfbb773710d3d17d7f753a5403e08/backend/apps/matching/models/participant.py#L63-L66 something where we first filter for 'p_type' and then filter for 'is_approved'? Alternatively, we would have to also name the groups according to the names in the config.

But we would have similar problems for if not approver.has_perm(

[Baschdl]

@bjrne isort is blocking this one like #175 but possibly one other formatting issue

[feeds]

• [x] change the urlconverter
• [x] urlencode for url_name