use obo-label-names in sentences as well

This is solved by just calling set_display_text! on the
sentences at the right time (after the display_name for
the symbol has been updated).
add migration to update display_text of obo

Add the migration the update the display_text
of already existing obo sentences.
set displaytext in sentence only if non-nil.
optimize set_display_text! body.
improve conciseness of the obo-text migration.
use method syntax for blocks where possible.
Merge pull request #1123 from ontohub/1099-label_symbols_in_obo_sentences

1099 label symbols in obo sentences