Include a version string in sentry reports

[DMRobertson]

The following is the sum total of all the testing I've done:

$ python
Python 3.9.7 (default, Aug 30 2021, 00:00:00) 
[GCC 11.2.1 20210728 (Red Hat 11.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sydent.util.versionstring
>>> sydent.util.versionstring.get_version_string()
'sydent@2.4.6 (b=dmr/version-in-sentry,05e1fa8,dirty)'

[DMRobertson]

Will open a second PR with changes for these @anoadragon453

[DMRobertson]

With a specific commit hash checked out, b=HEAD wasn't so useful. But it's good enough.