Document USE_LDAP and ignore .env

[orenyk]

I realized that I left out the USE_LDAP environment variable when I set up .env.example. We're also tracking the .env file in Git, which isn't really good practice. Therefore, we should do the following:

• document USE_LDAP in .env.example
• document USE_LDAP in the configuration wiki page
• document LDAP stuff in the Heroku wiki page
• properly ignore .env
• move USE_LDAP to secrets.yml?
• move CAS_AUTH to secrets.yml?

The last two point occurred to me as I thought about removing the .env file... to be honest, we shouldn't really need to copy both a secrets.yml file and a .env file to run the app locally. I'm happy to use a Rails secret for both instead of the raw environment variables, any thoughts?

[orenyk]

I'm going to merge this into #275 since I still have some cleanup to do there.

[orenyk]

Ok, just finishing up the removal of .env from Git; from now on new users should copy the .env.example file and uncomment the example lines as desired (for LDAP and CAS). Other configuration parameters will be defined in secrets.yml.