CHANNEL build property to autoselect getdown appbase location for release, develop, local and bamboo builds

The getdown appbase that gets set by gradle getdown currently defaults to
http://www.jalview.org/getdown/TEST/ (or something like that).

This means that any compilation done with
gradle getdown
or even
gradle installers
defaults to this TEST channel.
If you try and launch Jalview using getdown then the Jalview you just compiled gets overwritten by the version languishing in the TEST channel (this was fine at first but isn't now).

Proposal:
Setting a CHANNEL property (e.g. with gradle getdown -PCHANNEL=...) overrides all other appbase guesses and sets the appbase to
http://www.jalview.org/getdown/CHANNELNAME/JAVA_VERSION

If CHANNEL has not been set then the getdown task (or some other task in build.gradle) checks whether it is compiling on the build server (jv-bamboo -- this already happens for the install4j tasks).
If it is then it can default to an appbase assuming the getdown artifacts will be built, and use, e.g.

https://builds.jalview.org/artifact/JB-JGP/shared/build-15/getdown-channel/11

if it isn't then it can assume it is being built on a developer's machine and default to using the file://absolute_path_to_jalviewdir_used/getdown/website/<JAVA_VERSION> appbase that is currently used when you set -Pgetdown_local=true

All builds should also contain a launch.jvl file with just the appbase value.