#562 - added button to test connection if backup strategy is "git remote"

...in/java/de/deadlocker8/budgetmaster/backup/GitHelper.java

...deadlocker8/budgetmaster/settings/SettingsController.java

src/main/resources/languages/base_de.properties

src/main/resources/languages/base_en.properties

src/main/resources/static/js/main.js

src/main/resources/static/js/settings.js

src/main/resources/templates/settings/settings.ftl

src/main/resources/templates/settings/settingsMacros.ftl