backend: add absolute path to template links

fixes: #502

Add a new template function called `absPath` that converts relative
paths to absolute paths based on the issuerURL. The template function
can be used in templates like:
````html
<a href="{{ .MyRelativePath | absPath }}">
````

cmd: update flag defaults for dev mode

Updates flags in dex-worker for no-db mode to work with default static
files.

*: depricate --email-from flag and move to email config files

Add groups scope and LDAP implementation

connector: remove erroneous logging statement

Use "plain" auth if and only if username and password are provided.

Parse port from "host" field if it's provided.

docs: fix various typos & Markdown lint issues

Documentation: clean up release documentation

Documentation: minor fix

return 409 for duplicate client ids

Clean up LDAP Connector

* Remove some unlikely to be used fields to help configurability.
  * Combined "serverHost" and "serverPort" into "host"
  * Remove "timeout" (just default to 30 seconds).
  * Remove "maxIdleConn" will add it back if users feel the need
    to control the number of cached connections.
  * Remove "trustedEmailProvider" (just always trust).
  * Remove "skipCertVerification" you can't make this connector
    ingore TLS errors.
* Fix configs that don't search before bind (previously broken).
* Add more examples to Documentation
* Refactor LDAPPool Acquire() and Put() into a Do() function which
  always does the flow correctly.
* Added more comments and renamed some functions.
* Moved methods on LDAPIdentityProvider to the LDAPConnector

return 409 status code on duplicate email errors

Fix docker-build script

*: update go-oidc to shorten length of key IDs

*: update mailgun-go to remove dependency on deleted simplehttp pkg

build with go 1.6 and actually push to quay

scope: don't validate empty scopes

If an empty scope is somehow passed along, it shouldn't be validated
when checking refresh token scope.

Allow specification of client_{id, secret} in admin api

You can specify your own client ID and secret!