Add more human-readable GitDescribe item to the status page

[mostynb]

bazel-remote has included a GitCommit field in the /status page for some time, but the git ref SHA1 value is not the most readable. By adding a new GitDescribe field with the output from git describe --tags it is easier to see which version of bazel-remote is running.

For details of the values this field can hold, see: https://git-scm.com/docs/git-describe

Relates to #436.