[UI] Remove Image button (for custom images) is presented to non-Admin users

When removing a custom image using the "Remove Image" link on the "Images" page - no feedback is given in the UI if this action fails due to lack of permission. After clicking the Remove button to confirm the action, the page stays the same with no feedback and the image is still listed.

An error is logged to the server log but there is no client feedback.

Additionally, the Remove Image link should probably not be presented in such a case.

= Steps to reproduce =

Tested on Bionic, MAAS 2.4.2-7034-g2f5deb8b8-0ubuntu1 from bionic-updates

(1) Upload a custom image using the MAAS CLI as a MAAS administrator
    maas admin boot-resources create name=windows/win2012r2 architecture=amd64/generic content@=./path/to/file (you can just use a dummy file)
(2) Create a non-admin user, and login to the web interface with it
(3) Browse to the images tab and select the "Remove Image" link
(4) Observe no change in the UI, reload page, the image is still there
(5) Check regiond.log and observe the error below

2018-10-17 12:15:39 maasserver.websockets.protocol: [critical] Error on request (8) bootresource.delete_image: Permission denied.
 Traceback (most recent call last):
   File "/usr/lib/python3.6/threading.py", line 864, in run
     self._target(*self._args, **self._kwargs)
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 850, in worker
     return target()
   File "/usr/lib/python3/dist-packages/twisted/_threads/_threadworker.py", line 46, in work
     task()
   File "/usr/lib/python3/dist-packages/twisted/_threads/_team.py", line 190, in doWork
     task()
 --- <exception caught here> ---
   File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 250, in inContext
     result = inContext.theWork()
   File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 266, in <lambda>
     inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 122, in callWithContext
     return self.currentContext().callWithContext(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 85, in callWithContext
     return func(*args,**kw)
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 883, in callInContext
     return func(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 232, in wrapper
     result = func(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 756, in call_within_transaction
     return func_outside_txn(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 563, in retrier
     return func(*args, **kwargs)
   File "/usr/lib/python3.6/contextlib.py", line 52, in inner
     return func(*args, **kwds)
   File "/usr/lib/python3/dist-packages/maasserver/websockets/handlers/bootresource.py", line 834, in delete_image
     assert self.user.is_superuser, "Permission denied."
 builtins.AssertionError: Permission denied.