Add build tag "heroku" a synonym for "sqlite cert pam"

Describe the bug
Same issue as #2457 #685 #1785

packager.io uses heroku buildpacks to compile the application. When the buildpack runs, the tag set "heroku" is used in place of "sqlite cert pam"

To Reproduce
Fetch and install any package from packager.io and attempt to use either a SQLite database, PAM authentication, or self-signed certificate generation

Expected behavior
packager.io builds should have the same feature set as binary builds

Additional context

• Gogs version (or commit ref): 12680616ddd51c2f37682202854d6753dd5de602
• Git version: N/A
• Operating system: tested with centos 7, but should exist on any platform
• Can you reproduce the bug at https://try.gogs.io:
  • Yes
  • No
  • Not relevant

Start by locating the build-tag configuration that defines the "sqlite cert pam" tag set and compare it with the tag used by packager.io's Heroku buildpack. Done means the "heroku" tag provides the same SQLite, PAM, and self-signed certificate features as binary builds.