webadmin: replace column header with icon

- Replaced the comment header text with the comment icon.
- Added the comment text as a tooltip to the icon.
- Moved the comment column in the following grids:
  - Clusters
  - Data centers
  - Hosts
  - Networks
  - Pools
  - Storage
  - Templates
  - VMs

Change-Id: If4d7165565e46f351c8880753c1e3aaa829c6cd5
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1035566
Signed-off-by: Alexander Wels <awels@redhat.com>