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.
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
fix(database): More robust sql script execution.

Implemented tests for various formats of SQL input and fixed a bug that caused
text inside SQL strings to be interpreted as comments. Specifically, "INSERT
INTO x values ('some string -- not a comment');" is now supported. Also added
support for comments prefixed with "# ".
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
Merge pull request #7298 from juho-jaakkola/likes_undefined_delete_button

chore(likes): removes warning about undefined delete_button variable
Merge pull request #7294 from stianlik/1.9

fix(database): More robust sql script execution.
Merge pull request #7301 from PerJensen/search_fix

fix(aalborg_theme): display search when logged out
Merge pull request #7282 from jeabakker/discussion_subscriptions

fix(discussion): discussion replies respect previous subscribers
fix(db): query cache properly handles more callable types

The former query cache did not generate unique ids for callbacks. Notably all
array callbacks were considered the same.