metadata: add new Translation app field for URL for contributions

This is a field requested from a bunch of translators so they can easily
find where to translate apps:
https://forum.f-droid.org/t/translation-field-for-app-entries/1403/5

tests/metadata/dump/com.politedroid.yaml

@@ -29,6 +29,7 @@ RepoType: git
 RequiresRoot: false
 SourceCode: https://github.com/miguelvps/PoliteDroid
 Summary: Calendar tool
+Translation: ''
 UpdateCheckData: null
 UpdateCheckIgnore: null
 UpdateCheckMode: Tags

tests/metadata/dump/org.adaway.yaml

@@ -52,6 +52,7 @@ RepoType: git
 RequiresRoot: true
 SourceCode: https://github.com/dschuermann/ad-away
 Summary: Block advertisements
+Translation: https://www.transifex.com/dominikschuermann/adaway
 UpdateCheckData: null
 UpdateCheckIgnore: null
 UpdateCheckMode: Tags

tests/metadata/dump/org.smssecure.smssecure.yaml

@@ -49,6 +49,7 @@ RepoType: git
 RequiresRoot: false
 SourceCode: https://github.com/SMSSecure/SMSSecure
 Summary: Send encrypted text messages (SMS)
+Translation: https://www.transifex.com/silence/silence
 UpdateCheckData: null
 UpdateCheckIgnore: null
 UpdateCheckMode: Tags

tests/metadata/dump/org.videolan.vlc.yaml

@@ -51,6 +51,7 @@ RepoType: git
 RequiresRoot: false
 SourceCode: http://git.videolan.org/?p=vlc-ports/android.git;a=summary
 Summary: Media player
+Translation: ''
 UpdateCheckData: null
 UpdateCheckIgnore: null
 UpdateCheckMode: Tags