Add permissions check at start of editing

[mararad]

When starting editing, test writing a randomly named attachment and deleting it. If either fail, tell the user to go and read a knowledge base article that I need to write...

If the initial write fails with a 302 returned, it's this one
https://groups.google.com/forum/#!topic/atlassian-connect-dev/VbmNcgvRFe4.
We'll need another KB article to point users at to explain to add draw.io user to jira-software-users, or whatever the base product is.

Similar to what we do in P2 JIRa, we could simply check for CREATE and DELETE permissions. It can be done with a single rest call.
/rest/api/2/mypermissions