Add default icon to OAuth apps

Add default icon to OAuth apps

When MySQL runs in strict mode, it doesn't seem possible to add a new OAuth app.

This is because:
 * The 'icon' field has no defaults and also has NOT NULL constraint.
 * GS uploads/sets the icon[1] (if provided) after the SQL insert[2]

The easiest fix seemed to be to provide a default icon value.
This change sets the default to the  "blue peg" default avatar from the base theme.

[1]
https://git.gnu.io/gnu/gnu-social/blob/7f30e614027fa27e83ecc32eb5443ee82d03f0f7/actions/newapplication.php#L176

[2]
https://git.gnu.io/gnu/gnu-social/blob/7f30e614027fa27e83ecc32eb5443ee82d03f0f7/actions/newapplication.php#L186