The two methods `StreamEntriesHelper#stream_link_target` and
`StreamEntriesHelper#acct` are based on checking whether we are running
in an embedded view.

This adds some test helper code to make the testing easier. We extracted
some "magic strings" to constants to lower the coupling in the specs.

The code that generates CSS is based on a lot of boolean conditions.
The possible combinations of these grows exponentially as we add more
conditions.

Since most of the code is conditional on a single boolean, we tested the
following:

1. All `false`
2. All `true`
3. Each individual flag set to `true`

The methods tested are:

* `StreamEntriesHelper#style_classes`
* `StreamEntriesHelper#microformats_classes`
* `StreamEntriesHelper#microformats_h_class`

We used some random Arabic characters to test that the various RTL
conditions got triggered.

* Remote spec for non-existent entry_classes helper method

This method no longer exists, and is handled by a local variable in a partial
instead.

* Remove spec for non-existent Account#ping! method

Removes:

- avatar_for_status_url
- relative_time
- reblogged_by_me_class
- favourited_by_me_class

* Remove unused helper files

* Add coverage for application helper

* Add coverage for StreamEntriesHelper #display_name