Encompasses designs from
 - https://gitlab.com/gitlab-org/gitlab-ce/issues/23206#note_43355199
 - https://gitlab.com/gitlab-org/gitlab-ce/issues/29985#note_30417407

Adds a test for flattenedFiles
Changes the data method to not be an arrow method
Various other review fixes

fixed previous directory showing when it shouldnt

fixed eslint

Updated as best as possible, a lot of tests still change the store
and never reset the state back which can cause some issues with other
tests.

This moves away from storing in a single array just to render the table.
It now stores in a multi-dimensional array/object type where each entry
in the array can have its own tree. This makes storing the data for
future feature a little easier as there is only one way to store the
data.

Previously to insert a directory the code had to insert the directory
& then the file at the right point in the array. Now the directory
can be inserted anywhere & then a file can be quickly added into this
directory.

The rendering is still done with a single array, but this is handled
through underscore. Underscore takes the array & then goes through
each item to flatten it into one. It is done this way to save changing
the markup away from table, keeping it as a table keeps it semantically
correct.

This reverts merge request !14768

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/38871