Add `min_uid` and `max_uid` configuration values.

[JOT85]

aad-auth assigns users to UIDs which are too large for some common software.

There are many reports of this problem, notably relating to xdg-desktop-portal-gnome not working:

• In https://github.com/ubuntu/aad-auth/issues/278, screensharing does not work because the portal is not loaded.
  • We have also had this problem.
• In https://github.com/ubuntu/aad-auth/issues/200, applications take excessively long to open.
  • We have experienced this issue with a variety of apps, including the nmapplet, gnome-terminal, and chromium.
• In https://github.com/ubuntu/aad-auth/issues/441 brings up exactly this issue, but hasn't had a response.

Adding a min_uid and max_uid configuration option allows the user to specify the range in which UIDs should be generated, thereby enabling admins to cap the UIDs at a range which works with most software.

To prevent existing installations from changing their behaviour, the default values, when the parameters aren't specified in the configuration file, remain at 100000 and math.MaxUint32, however the config template now explicitly sets the values to values which play nicely with xdg-desktop-portal-gnome, in an attempt to give new users a better experience.

Also, when a collision is found, instead of only incrementing the UID, which may overflow and end up as UID 0 (root!!!), we instead wrap around only within the specified range.

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