add char limits for text: Name=30, Video=256, WhatsNew=500

Since this info can come from automated sources, this prevents giant blobs
from being mistakenly stuck in these fields.  It also brings it inline with
the standard lengths other popular app stores use.

https://support.google.com/googleplay/android-developer/answer/113469?hl=en
https://stackoverflow.com/a/20276565

fdroidserver/common.py

@@ -93,8 +93,11 @@ default_config = {
     'keystore': 'keystore.jks',
     'smartcardoptions': [],
     'char_limits': {
+        'Name': 30,
         'Summary': 80,
         'Description': 4000,
+        'Video': 256,
+        'WhatsNew': 500,
     },
     'keyaliases': {},
     'repo_url': "https://MyFirstFDroidRepo.org/fdroid/repo",