Replace safeURLName usage in code

[deepakchethan]

Overview

Replacing the usage of safeUrlName for designDocs with removed _design/ with a appUtils method getSafeIdForDoc which does the same internally. I have updated the code in places which was manually doing the same.

Testing recommendations

Current test cases should cover the regression testing.

GitHub issue number

Fixes #875

Checklist

• [X] Code is written and works correctly;
• [X] Changes are covered by tests;
• [ ] Documentation reflects the changes;
• [ ] Update rebar.config.script with the correct tag once a new Fauxton release is made

[deepakchethan]

Hello @Antonio-Maranhao @popojargo, Can you please review this PR or let me know how I can get it reviewed? Thank you in advance!