Changelog first line mod-updates behavior as its own field

[budak7273]

Currently the first few lines of the changelog are used as a summary for display on the Discord in the #mod-updates automated post. This behavior is undocumented.

Introduce a new required summary field to clearly take this purpose.

• The field should be character limited (length, # of newlines) to what the mod-updates post supports.
• Should field support markdown?
• Field should not allow the words version, changelog or a semversion (exact exclude list subject to change). This is to encourage people to write an actual summary instead of something restating the mod version again.
• When viewing a version page, the summary should be shown in its own box at the top
• Field could also be shown as a short summary on the Mod Versions page
• Existing mod versions without this field could either respond with empty string or the first few lines of the changelog to mimic old behavior