MWTemplateNode should serialise original HTML if unchanged

To help the selective serialiser we can return the original
HTML for generated content if it is unmodified.

As the output of toDomElements now depends on changes
to the dataElement we now have a 'modify' function in
the some test cases.

We also now have 'storeItems' to assert that the index-value
store is correctly populated and for loading values back
into the store for toDomElements tests.

Also make 'mw' an attribute and remove 'about' property.

Bug: 47394
Change-Id: I2bbb5d2d6a90c4eb87fa129671112c92a9b931e7
Merge "MWTemplateNode should serialise original HTML if unchanged"