Update entry points and target webpack configs

[unDemian]

Summary

There are a couple of devex enhancements that are part of this PR

• There is a new ./src/api.js file which acts as a single exporter of the Happychat client api and it is used for all targets' entry points.
• An entry point without exposing the api to the window was added in order to have a more generic structure preparing for other targets like npm
• Build npm scripts have been added for all targets's css and js also development scripts
• Target dist was renamed to browser because it does expose the happychat api to the window object (dist can be too generic especially as we add npm as a target also)
• The config system now accepts development.local.json similar to calypso and other hc products so that developers can easily override configuration fields without comiting them.
• local CSS will be used in development
• The built css/jss files are gitignored now

Testing

• Added docs should make sense
• npm scripts should work as advertised
• standalone chat should work as before

[oandregal]

Hey @unDemian these days my bandwidth to take more work is limited because I'm focused on finishing some tasks before going AFK in a couple of days. This PR does a lot of unrelated things and requires a lot of my brain power. In other situation, I wouldn't mind putting more reviewing hours so you don't have to, but I'm afraid I don't have them at the moment. I could offer some reviewing help to move this forward if this was divided into smaller PRs that make only one thing so we can land them separately as they're ready.

[unDemian]

Hey @unDemian these days my bandwidth to take more work is limited because I'm focused on finishing some tasks before going AFK in a couple of days. This PR does a lot of unrelated things and requires a lot of my brain power. In other situation, I wouldn't mind putting more reviewing hours so you don't have to, but I'm afraid I don't have them at the moment. I could offer some reviewing help to move this forward if this was divided into smaller PRs that make only one thing so we can land them separately as they're ready.

No worries, I'll split it into smaller PRs.

[unDemian]

Will close this in favor of smaller PRs
#176
#177
#178
#179
#180