resolves #2814
builds off #3474 (I'll rebase after that one is merged)

Description

dbt init will scaffold ~/.dbt/profiles.yml (if missing) using an adapter-specific profile. Neat! It accepts an --adapter flag to control which one. If unspecified, it would default to redshift — even if the Redshift adapter isn't installed. Not so neat.

Changes:

• Remove redshift as default for --adapter. If --adapter is not specified, use _get_adapter_plugin_names to grab the set of installed adapters. Pick the first one and use it. (That method is currently in dbt.version, we could move it into dbt.utils.)
• If sample_profiles.yml is missing in the selected adapter, don't raise a runtime error, just skip writing profiles.yml. Do the same if no adapters are installed.
• I moved around some of the logging so it's directly tied to the action being performed, or skipped if the associated action is skipped.
• While here, I also updated the link to community slack to https://community.getdbt.com/. Previously, it was There's a link to our Slack group in the GitHub Readme (meh). I'm assuming that the new-ish community link is relatively stable, and that if it were to change, we'd do a good job forwarding traffic to account for older versions of dbt. (@annafil Want to check with you first, of course!)

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.