toaster: move MostRecentBuildsView to its own widget

toaster: move MostRecentBuildsView to its own widget

This view is specific to the builds dashboard rather than gernic api so
like ToasterTable and ToasterTypeAhead we class it as a widget as it has
a single purpose. Also clean up some flake8 identified issues.

Original author of the code moved in this commit is Elliot Smith.

Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>