fix: double-lock when multiple dashboards have the same UID

[weisdd]

Description

As it was described in #860, controller tries to acquire a second lock (and, thus, gets stuck) when there are multiple dashboards with the same UID and one of those gets deleted.

The issue is that Lock() is called from within the for loop, so the fix simply requires the lock to be moved outside of the loop.

Additional changes:

• Not sure why dashboard references with a particular uid would get removed for all namespaces, not just the one matching the delete request:
  => I fixed that, though, please, let me know if the behaviour was intentional;

• RemovePluginsFor() didn't have lock:
  => Added the lock.

Relevant issues/tickets

Closes: #860

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• This change requires a documentation update
• I have added tests that prove my fix is effective or that my feature works
• I have added a test case that will be used to verify my changes
• Verified independently on a cluster by reviewer

Verification steps

Follow the steps described in #860:

1. Deploy two dashboards with the same uid;
2. Remove of of those;
3. Reconciliation should not get stuck.

[NissesSenap]

This LGTM but I would like to get some feedback from @HubertStefanski or @pb82 since they got more history then I do.