The Theme constructor expects a triple (basename, logo, stylesheet),
which is passed by the User::get_themedata function.  But the three
properties are stored separately in the User object.  If they are
grouped together, then it will be easier to switch themes in and out
at a later date.

The three properties are combined under the 'institutiontheme' name,
to make it clear that they are defined by the user's institutions, and
to distinguish them from the user's own theme preference.

This requires a slight change to code for setting a view theme on view
creation, where only the basename property is allowed, because custom
themes with logos & stylesheets are not yet available as view themes.

Change-Id: Ie400a0f1c4a2172382dfaba14139f1b0b27f2120
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>