Deduplicated Media Storage, Tag Nav and Editing, XHTML-IM, and more

* Content Addressable media storage based on SHA-256 for deduplication
* Tag navigation UI when searching contacts
* Allow editing tags on contacts
* Do not show double notificatin during dialler-integration call
* Allow export/backup of an account where localpart is a large number
* Allow retracting messages
* Dump logs after every call (to make debugging easier)
* Allow keeping data on uninstall (to make switching builds easier)
* Starting private group with @cheogram.com JIDs asks if you want a group text
* New vector chat bubbles (thanks Adaoh!)
* New onboarding screen, beginnings of eventual flow
* Render XHTML-IM including inline media if it is Bits-of-binary
* Performance improvement for low-power devices during startup
* Fix race condition on legacy bookmark sync
* Allow long press to copy any link in a message, not just the first one