Distinguish between power=line and power=minor_line

Tagging distinguish between major power lines, which we map with
power=line and minor power lines, which we map with power=minor_line.
Clean up tests
Pass entity array to Tree#rebase
More tests
Rewrite tree logic

The main problem with the existing logic was that it
did not update the extents of relations when previously incomplete members were loaded.

It was also overly stateful; the various queues and flags
are no longer required.

Fixes #1928.
Fixes #1540.
Use assumed values for access placeholders (fixes #1924)
Remove unused
Lower match score of generic presets (fixes #1933)
Calculate background source name (fixes #1935)
Add office presets (fixes #1875)
Add alternate terms suggested based on en_GB translation
Merge pull request #1936 from pnorman/misc_translations

Add alternate terms suggested based on en_GB translation
Add name-suggestion-index sources
Remove repetition of entity IDs in tests

There were several places where the key and entity id
didn't match up.
Make the Graph#rebase argument an array
Add name autocomplete
Use .low-zoom class rather than hard-coding z16
Limit suggestions to three 'closest' matches
Add SuggestNames util
Add tests for suggestNames