Add `gdk pristine` command

What does this Merge Request do?

⚠ This MR relies upon !1845 (merged) (and !1844 (merged)).

This MR introduces the gdk pristine command which does the following:

• Removes all files/directories in GDK tmp/ (and then restores files as necessary)
• Runs go clean -cache
• Ensures GDK Ruby gems are installed and pristine (bundle install and then bundle pristine)
• Ensures gitlab/ Ruby gems are installed and pristine (bundle install and then bundle pristine)
• Ensures gitaly/ruby Ruby gems are installed and pristine (bundle install and then bundle pristine)
• Removes all files/directories in gitlab/tmp/ (and then restores files as necessary)
• Runs yarn clean in gitlab/
• Adds suggestion to run gdk pristine if something goes wrong

Once !1174 (merged) is merged, I'll include running this too and look to combine the too into simply gdk pristine. 🙂

Example output:

$ gdk pristine
=> Cleaning GDK tmp/ ..
=> Cleaning go cache..
=> Ensuring GDK Ruby gems are installed and pristine..
--snip--
=> Ensuring gitlab/ Ruby gems are installed and pristine..
--snip--
=> Ensuring gitaly/ruby/ Ruby gems are installed and pristine..
--snip--
=> Cleaning gitlab/tmp/ ..
=> Cleaning gitlab/ Yarn cache..
--snip--
Successfully ran 'gdk pristine'!

Closes #843 (closed) #1139 (closed)

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.