Remove data-placeholder from template

Remove data-placeholder attribute from admin settings template.
Otherwise changing the quota fails in ownCloud 6.

Fixes #52.
Beautify admin page for OC6 release

Change heading from label to h2 and add title hint to sync quota with
size zero.
Read updated language strings
Change heading from legend to h2 on settings page
Update CHANGELOG and version for v1.2.2