Make url_prefix optionnal in url map

[tbille]

Summary

Make the url_prefix optional in the url_map

The URL map can look like this now, if url_prefix==/docs:

# URLs

[details=Mapping table]

| Topic | Slug |
| -- | -- |
| <discourse_url> | /toto |
| <discourse_url> | tata |
| <discourse_url> | /docs/titi |
[/details]

This means that:

# URLs

[details=Mapping table]

| Topic | Slug |
| -- | -- |
| <discourse_url> | /toto |   -> /docs/toto
| <discourse_url> | tata |   -> /docs/tata
| <discourse_url> | /docs/titi |    -> /docs/titi
[/details]

QA

• add this module to charmhub
• Run charmhub with module and go on:
• http://0.0.0.0:8045/mattermost/docs
• Compare with URL map here: https://discourse.charmhub.io/t/mattermost-documentation-overview/3758