Extlinks review

Similar to openmicroscopy/ome-documentation#187, this PR reviews extlinks used across documentation and allow to point  source code at the daily merge branches:

- **Source code**: the fork of the source code linked by the :source:, :bfreader: etc aliases can now be specified with SOURCE_USER while the branch still can be specified with SOURCE_BRANCH. Can be tested with the daily pushes to snoopycrimecop/bioformats as:
```
SOURCE_USER=snoopycrimecop SOURCE_BRANCH=merge/dev_4_4/latest make clean html
```

- The extlinks dictionary list has been reviewed and mirrors the dictionary list of ome-doc.git whenever applicable.