Add feature-freeze configuration option

[Riscky]

Closes https://github.com/channable/hoff/issues/243

This adds a configuration option, called featureFreezePeriod.

In the configured feature freeze period:

• Normal merge commands are blocked;
• Instead, merge hotfix commands are allowed;
• merge and (tag | deploy) becomes merge hotfix and (tag | deploy)
• Using merge hotfix outside of the configured feature freeze period is not allowed.

A few choices I made here:

• The default on Friday suffix is not necessary during a feature freeze, to keep it simple;
• For the retry command, neither on Friday nor hotfix is necessary to specify;
• The feature freeze period is configured globally, not per project.

[Riscky]

@OpsBotPrime merge

[OpsBotPrime]

Pull request approved for merge by @Riscky, rebasing now.

[OpsBotPrime]

Failed to rebase, please rebase manually using

git fetch && git rebase --interactive --autosquash origin/master riscky/feature-freeze-feature

[Riscky]

@OpsBotPrime retry

[OpsBotPrime]

Only approved PRs with failed builds can be retried..

[Riscky]

@OpsBotPrime merge

[OpsBotPrime]

Pull request approved for merge by @Riscky, rebasing now.

[OpsBotPrime]

Rebased as 74ab82f7b4d03929408f3428e922bd0e66ada414, waiting for CI …

[OpsBotPrime]

CI job :yellow_circle: started.