Add support for EncryptedRichText

[jdhollis]

Hi, all.

I’ve been building a new product, and I want to use Lexxy as my default rich text editor.

I’m encrypting almost everything in my app by default, but it seems the temporary rendering method did not support encrypted ActionText fields.

I know this will likely be sorted upstream once ActionText supports adapters, but in the meantime, I need Lexxy to work with encrypted fields.

Switching to to_html seems to work, and the test suite is all green. I’ve added one test to explicitly hit the encrypted field path. Otherwise, I’m not entirely certain what other tests would be valuable here. (I was mostly concerned whether I broke attachment functionality.)

This commit can likely be trimmed down if we don’t need to do testing beyond the fact that Lexxy can handle encrypted fields.

Also, not sure how to handle encryption-related secrets in a friendly manner for testing purposes in this context. Hard-coding in config/application.rb offends my sensibilities. But this feels like a higher-level CI decision beyond the scope of this PR.

Your feedback is welcome.