Make it clearer what the rummager rake tasks do

It wasn't clear that these tasks index more than just "documents". In
fact they either index detailed guides, or everything else that is
searchable.
Rename misleading methods

At some point in the distant past, Whitehall.government_edition_classes
did actually return only Edition classes. Now, it returns all classes
that are searchable and should be indexed in the "government" index,
i.e. all searchable classes except DetailedGuide. I've renamed these
methods to avoid confusion in the future.
Start role year selects at this year, not 1700

* Starting from 1700 and having to scroll all the way down to 2014, or
even 2019, is painful
* Restrict date entries to current year rather than current year + 5
which seemed to be the default
Indicate that change notes are public

* Research showed that it wasn’t clear whether this note was internal,
like remarks are, or visible to the public
Confirm announcement is published

* The button action says “publish”, this should be affirmed in the
flash message
Refine role copy

* The second part of the sentence was hard to understand
Merge pull request #1656 from alphagov/rewrite-code-for-clarity

Rewrite code for clarity
Merge pull request #1658 from alphagov/improve-role-date-selection

Improve role date selection
Handle CSV preview issue

This improves the handling of badly encoded CSV files so it fails more
gracefully when encountering dodgy characters.
Mark non-public methods as private
Merge pull request #1655 from alphagov/csv-preview-robustness

Handle CSV preview issue
Merge pull request #1661 from alphagov/public-change-note

Tweak statistics announcement copy
Remove unused checked assign

* Checked variable isn’t being used anywhere
Lock & hide display date fields if confirmed date

* When the date is confirmed, the display must be exact — users were
confused when they selected a date, the radio option changed but they
were still able to switch it back to one or two month precision
* Instead, switch to exact, lock the form field and hide the one and
two month options
Move release date fields into partial

* Stop repeating fields in new announcement and new date change forms