Searching for references should take sentence casing into account

[jzohrab]

Currently, searching for a term reference will only find sentences that match the case. e.g. if I search for DOG, I don't think it will find references with dog ... at least, it won't work for sentences with funny casing rules like Turkish.

To do:

• add test to finding references to illustrate the problem, e.g. add Turkish test
• add sentences lowercase field to sentences table
• add some kind of data cleanup job to fix the cases for existing sentences -- might need to do this piecemeal, depending on the size of the database, e.g. fix 10K sentences at a time. Optionally, could do a check at startup and write some kind of progress to the console ... Need to check performance.
• add sentence downcasing to whatever job loads the table

[jzohrab]

Phew, merged to develop. Seems fine in my dev env, tested with mecab load and unload as well.

Launch docs: may take a while to run on your machine, depending on how much data and the type of langs you're studying.

[jzohrab]

Launched in 3.7.0