docs(actions): Migrated actions section from old Getting Started docs and cleaned up related sections.

Refs #6812.
docs(events): Updated event list and cleaned up existing event docs.

Refs #6812.
docs(hook): Updated hooks docs.

Refs #6812
docs(all): Added getting started for developers.

Refs #6812
docs(all): Cleanup docs.

Refs #6812.
docs(admin): Migrated Getting Started guide from wiki.

Refs #6812.
docs(faqs): Started migrating some FAQs.
chore(docs): minor change to test the PHP syntax highlighting

Refs #6700
Merge pull request #7249 from juho-jaakkola/highlight_test

chore(docs): minor change to test the PHP syntax highlighting
docs(js): Adds docs for more JS functions, improves docs for elgg.echo
Merge pull request #7257 from mrclay/js_docs

docs(js): Adds docs for more JS functions, improves docs for elgg.echo
chore(i18n): update translations
chore(release): v1.9.3
Merge pull request #7268 from juho-jaakkola/release-1.9.3

Release 1.9.3
chore(tests): throw ElggUpgrade exceptions untranslated to get rid of library dependencies

ElggUpgrade was translating its exception messages which required
ElggUpgradeTest to include the languages library. Use of the libraries
is prone to cause dependency chains, so it's better not to translate
the exceptions at all for now.

Fixes #7276
Merge pull request #7277 from juho-jaakkola/dont_translate_upgrade_exceptions

chore(tests): throw ElggUpgrade exceptions untranslated to get rid of library dependencies
fix(i18n): allow core to load translations for a specific language on-demand

By default Elgg is loading only English and the language of the logged in
user. These two are not enough e.g. when we want to send a notification and
the recipient is using a third language. This fix makes it possible to load
translations for a specific language when necessary.

Refs #3151
fix(discussion): discussion replies respect previous subscribers

Existing subscribers to discussions were ignored when getting the
interested users for a discussion reply.

fixes #7281
fix(css): prevent select box from overflowing its container

Fixes #7290
docs(plugins): Information on activation/deactivation

Copied information on activate.php and deactivate.php from legacy documentation and linked to
older information on plugin skeleton.