Shorten time zone vertical tab summary

[NormPlum]

Description of the need

On the user account form, if the user can set their own timezone, the vertical tab where they set the timezone has a summary of the chosen timezone, but it's the full timezone string. This makes it very long and can span over 3 lines:

Proposed solution

Shorten the timezone since this is a 'summary' by displaying just the first part before the colon:

Alternatives that have been considered

None

Additional information

If the colon isn't found in the timezone name, it'll just return the full string.

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now shows shorter timezone summaries.

[izmeez]

I like this and it works for me.

[argiepiano]

The longer summary doesn't bother me; in fact I find it useful - seeing the expected day and time helps me catch misconfigurations.