SOLIDPOD: provide error dialog if user tries to share file to themself

[jesscmoore]

Description

As a user, I may accidentally try to share a resource to myself.

Either prevent this webid from being accepted in the webid input dialogs or provide an alert dialog advising them that they already have access, and to share to someone else.

Currently an unsuccessful snackbar popup shows up with list of possible error causes, asking them to review error console, which shows:

flutter: 💥 [GrantPermission] Exception occurred: Exception: Owner WebID cannot also be in third party access list

Why

The app should be designed to minimise the ability for the user to make a simple mistake.

Screenshot

Closing Criteria

Checklist for closing the issue:

• No errors from make prep
• All tests pass make qtest
• I cannot submit my own webid in the recipient webid input