Update naming of DSNP Announcement URI to DSNP Content URI

[wilwade]

Problem

Update the naming of DSNP Announcement URI to DSNP Content URI per https://github.com/LibertyDSNP/spec/issues/148

Double Checks:

• [x] Did you update the changelog?
• [x] Any new modules need to be exported?
• [x] Are new methods in the right module?
• [x] Are new methods/modules in core or not (i.e. porcelain or plumbing)?
• [x] Do you have good documentation on exported methods?

Change summary:

• Replaced isDSNPAnnouncementURI with isDSNPContentURI
• Replaced Type Name DSNPAnnouncementURI with DSNPContentURI
• Replaced buildDSNPAnnouncementURI with buildDSNPContentURI
• Replaced parseDSNPAnnouncementURI with parseDSNPContentURI
• Replaced InvalidAnnouncementUriError with InvalidContentUriError

Steps to Verify:

1. Search for Announcement URI in all forms and don't find any in the code