When handling an HTTP 409 for empty git repos, this:

    e.fp.read()

was yielding an empty bytestring. This caused us to pass through the
empty-repo handling code and crash with the HTTP 409.
Instead, we just do this:

    str(e)

and compare against that output, which is nearly as good.

The --check/-c option allows the name of a check class to be passed
in, case-insensitively:

    python -m migrate.repo_checks -c requiretriageteamaccess -c requireclacheck

Additionally, we improve the output in a few ways:
* bold the repo names
* make skipped checks cyan to distinguish them from steps
* print out checks that will be run at the beginning
* print check class names instead of class repr's

Finally, we add usage instructions to the module docstring.

We are preparing make the EnsureLabels checks the one-and-only
authoritative place for org-level labels. As part of this, we're
extracting the list of labels out of the Python script and into
a YAML file.

For repos that are already up-to-date with repo_checks, this change
should be a no-op.

Additionally, this cleans up the EnsureLabels code a bit so that it'll
be easier to add label-description support in the next commit.

Part of:
https://openedx.atlassian.net/wiki/spaces/COMM/pages/3695214629/openedx+org-wide+labels#1.-Proposal%3A-Store-labels-in-one-central-place

repo_checks currently normalizes label names and colors. With this
commit, it will also normalize the label description.

In interest of minimizing the effect of running the EnsureLabels check:
For labels that generally have already-established descriptions, we use
those. For labels that don't, we just use !!null for now, with the plan
of adding descriptions in the near future.

Part of:
https://openedx.atlassian.net/wiki/spaces/COMM/pages/3695214629/openedx+org-wide+labels#2.-Proposal%3A-List-of-starter-labels