1. Changed the 'delete' and 'eye' buttons to use an <a> link
tag so that it works without custom tabindex and shows a correct
mouse cursor. This also reduces the code slightly.
2. Changed the Cancel button on the dialog to use the mdl-align
class which is used in Atto dialogs. This results in the button
being centered, consistent with those dialogs.
If there is invalid data in the availability condition (note: this
should not happen, but can do in unusual situations, such as
specifying a date after 2038 in 32-bit PHP), this should not
leave the course so broken that users cannot edit the activity and
update it.
