Add theme tweaks for body text size

Fixes #891
Ensure JS macros return text
Extend the view widget to work with subtiddlers
First pass at an edition for translators working in the browser

The user interface needs some refinements but it shows the basic
principles.

The idea is that the translator would email their modified file, which
would then be merged into the core repo (some additional tools will be
helpful for this).

Fixes #984
small typo on WidgetMessage: tm-add-field

You can read FieldMangerWidget instead of FieldManglerWidget. A single L is missing, so the links are not working properly.
Dev docs update
Merge pull request #1005 from danielo515/patch-1

small typo on WidgetMessage: tm-add-field
More fieldmangler typos
Add an advanced search toolbar button
Release note update
Hide advanced search toolbar button by default
Adjust wording of theme tweaks
Clarify warning
Docs tweaks
Preparing for release of 5.1.4
Readme updates
Version number update for 5.1.4
Preparing for 5.1.5
Dev docs typo