Add missing variables to hook_civicrm_export

Fleshed out validate_callback doc.

Describe new variables due to dev/core#457

Fix incorrect sample filename

Update upgrade documentation to mention how to upgrade message templates

Add in documentation of as of 5.8 extensions can have examples loaded…

review.md - Split review standards into two buckets, "General" vs "Developer"

Add Civix example

Add "REF" (Refactor) as prefix for PR/git subjects

Under Hooks, switch the order of 'All Hooks' and 'Setup'

Currently we have 'All Hooks' followed by each of the categories of hooks
so it looks like 'Setup' is a hook category.  Its content is closer to 'Using Hooks'

Fix missing reference to pre/postJob hook files.

add more informatoin about creating an issue

CRM-21460 Add job execution hooks

Update setting metadata information

Remove reference to adding `CRM_Contact_BAO_Query::getGroupByFromSele…

dev/core#90 Add in documentation of the disabling and re-enabling of …

Update hook_civicrm_export

Describe new variables due to dev/core#457

Adding this clause has led to unreliable query results - better to give no information than to lead them wrong - many reports now disable & reenable full group by but that is referred to elsewhere.

Also better to override whereClause & selectClause

add 'CIVICRM_TEMP_FORCE_DURABLE'

Note working through this threw up some ambiguity about help_text vs help which I'll dig into as a follow up. I think there is further text on the page I need to review too about the settings form

dev/core#90 Add in documentation of the disabling and re-enabling of the ONLY_FULL_GROUP_BY sql mode for developers to use

Add in documentation of new hook hook_civicrm_postIPNProcess

Clarifications to buildkit path docs

Fix typo in hook_civicrm_links 

Remove unnecessary "are" in upgrade.md