MTControlCard: Use data attributes to keep track of current state

To indicate the state of whether the section is pure MT or something
changed or using the source text.

If section has data-cx-mt = true, section is pure MT
If section has data-cx-mt = false, section has manual edits
If section has data-cx-source = true, section uses source text as translation

Previously, the restore translation was not shown if user edited something.
With this patch, we show restore translation button whenever translator edit
something. This helps the translator to easily undo all edits and start from
MT.

Removed the MTControlCard.enableRestory object and
updateSection, showRestore, hideRestore methods

Change-Id: I76d49ed0c6803e61de5da7757e2cde910f8a5ae2
Merge "MTControlCard: Use data attributes to keep track of current state"