Implement pre-event signals for branch actions

[jeremystretch]

Plugin Version

v0.5.0

Proposed functionality

We currently have signals which fire at the end of a branch event (e.g. syncing or merging a branch). This FR proposes adding their pre-event complements, and renaming the existing signals to the form post_$action:

Pre-event signals

• pre_provision
• pre_deprovision
• pre_sync
• pre_merge
• pre_revert

Post-event signals

• branch_provisioned -> post_provision
• branch_deprovisioned -> post_deprovision
• branch_synced -> post_sync
• branch_merged -> post_merge
• branch_reverted -> post_revert

Use case

This will allow integrations to hook into the plugin both before and after branch events take place.

External dependencies

No response