[16.0][IMP] base_portal_type: use boolean field to define portal-group

[goreki]

This PR adjusts the circumstances under which a res.group becomes available for portal users: In addition to the magic module category in the data.xml of this module, a boolean field 'portal' can be set on a group to make it selectable for portal users. For backwards compatibility I have kept the module category, though.

This makes it also possible to put portal-groups into actual categories and then have them displayed in those categories on the res.users form view.

It addresses an old/stale/closed issue: https://github.com/OCA/server-backend/issues/232

Not sure if actively pinging the main contributor is best practice or frowned upon, but since this module was initiated by you, maybe you want to take a look :smile: @hbrunn

[OCA-git-bot]

Hi @hbrunn, some modules you are maintaining are being modified, check this out!

[goreki]

please add a sentence or two about the new flag in the readme, also explaining in which cases you'd want the flag. when done, squash your commits

thank you for all the feedback - I will integrate it as suggested during the weekend. Would you be okay with me adding myself as a contributor in the manifest?

When this PR is going to be merged, I will also take a look at migration to v17 and 18 (I see there is already a PR, which obviously does not have my adjustments included yet). I am not worried about any big changes - more about following the guidelines, so this will be another learning experience for me.

[hbrunn]

contributors go usually to https://github.com/goreki/server-backend/blob/16.0/base_portal_type/readme/CONTRIBUTORS.rst

[github-actions[bot]]

There hasn't been any activity on this pull request in the past 4 months, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 30 days. If you want this PR to never become stale, please ask a PSC member to apply the "no stale" label.