Change the file names expected for developer credentials

[erickoledadevrel]

Change the file names expected for developer credentials and stored tokens to better match the name of the downloaded file.

Developer credentials: client_secret.json => credentials.json
User credentials / token: credentials.json => token.json

Also:

• Switched to storing tokens in the local directory, instead of the home directory.
• Updated the Drive Activity API quickstart to bring it up to standard.
• Added back a check for command line execution that was lost in a previous refactor.
• Fixed glaring mistakes in the README files.
• Deleted quickstarts that were removed from the documentation.