active_group switch handles invalid group

[will-moore]

Fixes #259

To test:

• Go go /webclient/active_group/ (no ?active_group=1 specified)
• Should be simply redirected to /webclient (no error as on issue)

[snoopycrimecop]

Conflicting PR. Removed from build OMERO-python-superbuild-push#634. See the console output for more details.
Possible conflicts:

• PR Update jquery #180 will-moore 'Update jquery'
  • omeroweb/webclient/views.py
• PR Query string ids #225 will-moore 'Query string ids'
  • omeroweb/webclient/views.py
• Upstream changes
  • omeroweb/webclient/views.py

--conflicts Conflict resolved in build OMERO-python-superbuild-push#637. See the console output for more details.

[jburel]

on latest-ci:
https://latest-ci.openmicroscopy.org/web/webclient/active_group/ leads to

  File "/home/omero/workspace/OMERO-web/.venv3/lib64/python3.6/site-packages/omeroweb/webclient/views.py", line 364, in switch_active_group
    active_group = int(active_group)

TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'

<WSGIRequest: GET '/web/webclient/active_group/'>

on merge-ci redirect to https://merge-ci.openmicroscopy.org/web/webclient/