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.
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.
Merge pull request #7293 from stianlik/patch-2

Adds information on plugin activation/deactivation
Merge pull request #7291 from PerJensen/fix_select_overflow

fix(css): prevent select box from overflowing its container
chore(views): removed unused group link code in river summary view
chore(likes): removes warning about undefined delete_button variable
Merge pull request #7297 from jdalsem/unused-code-river-summary

chore(views): removed unused group link code in river summary view
fix(aalborg_theme): display search when logged out

Referencing #7169
docs(web_services): Documentation for Elgg 1.9

Updated function names to be compatible with Elgg 1.9. Information about
migration from 1.8 to 1.9.
Merge pull request #7292 from stianlik/patch-1

Documentation for Elgg 1.9