add Liberapay: field with username as data

Liberapay was originally included using a numeric ID, since they had
not yet finalized the public URLs.  Now it is a username.  So this
logic prefers the username in Liberapay: field, and keeps the old
LiberapayID: to ease migration.  LiberapayID: will not override
Liberapay:.  Clients are expected to prefer Liberapay: over LiberapayID:

tests/metadata/dump/com.politedroid.yaml

@@ -17,6 +17,7 @@ Disabled: null
 Donate: null
 FlattrID: null
 IssueTracker: https://github.com/miguelvps/PoliteDroid/issues
+Liberapay: null
 LiberapayID: null
 License: GPL-3.0-only
 Litecoin: null

tests/metadata/dump/org.adaway.yaml

@@ -40,7 +40,8 @@ Disabled: null
 Donate: http://sufficientlysecure.org/index.php/adaway
 FlattrID: '369138'
 IssueTracker: https://github.com/dschuermann/ad-away/issues
-LiberapayID: null
+Liberapay: null
+LiberapayID: '1234567890'
 License: GPL-3.0-only
 Litecoin: null
 MaintainerNotes: ''

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

@@ -37,6 +37,7 @@ Disabled: null
 Donate: null
 FlattrID: null
 IssueTracker: https://github.com/SMSSecure/SMSSecure/issues
+Liberapay: null
 LiberapayID: null
 License: GPL-3.0-only
 Litecoin: null

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

@@ -24,6 +24,7 @@ Disabled: null
 Donate: http://www.videolan.org/contribute.html#money
 FlattrID: null
 IssueTracker: http://www.videolan.org/support/index.html#bugs
+Liberapay: null
 LiberapayID: null
 License: GPL-3.0-only
 Litecoin: null