Data migration to strip leading and trailing blanks in Species fields

Some fields in previously-imported species objects have leading or trailing spaces,
causing false merge conflicts when trying to re-import species.

This data migration strips leading and trailing spaces from all Species fields in all instances.

Going forward the importer will ensure that all incoming Species fields are stripped.

Fixes OpenTreeMap/clients#73
"Merge Required" tab shows controls for resolving diffs and "solving" a merge

* Set up context for merge table (including unique names & ID's for radio buttons)
* Currently showing the merge table below its associated table row; could be changed to a popup.

Bug fixing and refactoring in Species validation, including:
* Filter Species by instance when matching
* Extract function `_prepare_merge_data` and rewrite
* Don't show diffs for a field if the matched Species value is empty (as it will be for most fields).
* If all diffs are with such empty values, don't require a merge
* Add tests for row status and merge data

Also:
* Fix layout problems in main `imports.html` template
* Rename errors.MERGE_REQ as errors.MERGE_REQUIRED
* Remove errors.TOO_MANY_SPECIES; it's subsumed by errors.MERGE_REQUIRED

Also, remove everything related to USDA code -- because it doesn't match OTM code like we thought.

We thought that OTM codes matched USDA codes except when there's a cultivar. But that's not the case. For example:
    Silver maple:  OTM code "ACSA1", USDA code "ACSA2"
    Sugar maple :  OTM code "ACSA2", USDA code "ACSA3"
    Willow acacia: OTM code "ACSA3", USDA code "ACSA10"

So matching USDA code introduces false matches (and was a marginal feature to start with) so it's now gone.