Getdown launcher to style the splashscreen depending on the CHANNEL

As referred to in comment https://issues.jalview.org/browse/JAL-3289?focusedCommentId=23427&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-23427 in JAL-3289:

Change getdown to style the splash screen so it prominently adds "Archive", "Develop", "Build" etc under the Jalview name (for example), depending on the channel that the launcher is opening.

Currently there is no good way to work out which channel is being used (other than looking at the URL of the appbase [maybe that would be sufficient! appbase.startsWith: "https://www.jalview.org/getdown/release", "https://www.jalview.org/old/", "https://builds.jalview.org/", "https://www.jalview.org/getdown/develop" ...]), but a CHANNEL key-value could be added to each channel's getdown.txt file.

First attempt at styling using altered background images uses a light grey Fira Code Light text below and left aligned with the word Jalview in the splash screen image. Fira Code is open.