Ability to add user texts

[tfnribeiro]

Currently, only teachers are allowed to upload their own texts.

However, their could be situations where a user might want to read a text they found, so providing them with a way to do this could allow us to have more ways of users discovering and interacting with their own content.

We also discussed the possibility of using Text-Generation to generate a text about a specific topic the user might interested in, including vocab for their level.

Things to consider:

• What to do if users want to upload too many texts? We should ensure users can't abuse the system, for example limited number of uploads by day / week.
• Private content, how to ensure users do not upload sensitive information, and if they do - can we quickly delete it / warn the user before they submit?
• Would this be fixed by allowing the extension to work on any highlighted text in the web? (https://stackoverflow.com/a/4379141)