TM: Filter out units based on their length

The Levenshtein distance is at least as much as the difference in string
length, so use this fact to filter out units that are impossible to have
the desired similarity.
TM: Add support for project icons

The icons are served from
"settings.PODIRECTORY/<project>/.pootle/icon.png".
Docs: update TM doc for local TM
TM: Populate the TM when upgrading from older versions

This is only needed in the pre-local TM versions, since adding new
translations will automatically added TM entries.