Add opt-in option to work with E2EE encrypted files in Web Interface

[sorbaugh]

The problem:

Currently, working with E2EE only works with clients. There should be an opt-in feature (disabled by default) to be able to work with encrypted files on the web interface as well

First step: read only

Since this would be a bit of work to implement, we should start with the following (avoiding anything that involves writing metadata as a first step):

• Settings entry to enable viewing encrypted files (app settings, not admin). Add a warning about limitation of features. ✏️ (text)
• Support browsing and viewing of encrypted files
• Add ways to share the mnemonic with the server
• Open office files in view only mode? would this be possible @juliusknorr?
• Possibly an icon somewhere in the files list file item that shows that the file is encrypted ✏️

[joshtrichards]

Some prior work on this: nextcloud/end_to_end_encryption#497