Updates to features, navigation, etc.

@BBonifield a few notes for you here:

There are a couple of new gems, one of which is dotenv-rails.  For running this app locally, a user should have a `.env` file with the required keys for certain things.  Here is a copy of my `.env` file locally: https://gist.github.com/createbang/76ae7f0433acd44201b9.  The only variables that I don't see on Heroku currently are the `KEEN_` values.  I just created an account for myself, but I imagine we'll want to have those filled in pre-deploy.

This commit is embarrassingly large, mostly because I was so caught up reacquainting myself with Rails that the work done started to get too expansive to break up, so I just put it in one commit.  This is uncharacteristic of my development style, just so you know.

I added a migration, and I can't remember if heroku's buildpack will automatically run any new migrations on deploy.  Just so you know a migration needs to occur.

I added a Heroku Scheduler task to the project, that will execute `bundle exec rake:accounts:project_leads:update_reminder_email` each day, but only send emails on Mondays.  I've never used Heroku Scheduler before, so it'd be good to get a second pair of eyes on this if you've had any experience getting it to work.

I think that's it!  Let me know if you want anything else adjusted or have any questions.  :boom: 

## Update: More (see: useful) information

#### What's this PR do?
Implements a number of feature additions to the application, including, but not limited to:

- Navigational links
  - direct link to project page, which shows all updates in created_at DESC
  - direct link to project lead, showing all projects separated by is_active status
  - direct link to specific update
  - if project lead (assigned as project lead or admin account)
  	- link to edit status
  - if admin account
  	- link to delete status
- Updates
  - new updates are sent to Keen.io for additional analysis
  - For rating fields (1-5), display delta next to rating
  - "show trends" button is graph pulled from Keen.io showing trends in that data point. (currently no data since I haven't imported previous data)
- Admin section
  - New functionality to inactivate user
  - New filter on manage user page to show all/active/inactive
- EKG
  - Navigation is not infinite loop anymore
- Scheduler Task
  - Rake task to run every monday at 9am, emailing all current project leads with reminder email to submit updates.

#### Where should the reviewer start?
Configure environment by following the instructions above re: `.env` file configuration, run `bundle install`, synchronize the database using the instructions in the updated README.md file, and then boot up the app. 

#### How should this be manually tested?
Test all navigational links.  Locally, the tester could update their Postgres user record to turn on and off is_admin boolean status to test conditional logic for controls.

#### Any background context you want to provide?
The `.env` strategy has proven successful for me in many applications, and I'm introducing it here.  Interested in feedback, though!

#### What are the relevant tickets?
Sprint.ly tickets in complete state for Heartbeat project.

#### Questions:
- This update includes one migration - are these run as part of the Heroku buildpack?
- Heroku needs to be updated with the appropriate `.env` values in the config vars - who does this?
@mhuggins refactored with this commit: https://github.com/quickleft/heartbeat/commit/d4c1e423b775c81b652e54bbf5cc46042cd27fb3