chore(bower): remove bower

[jaythomas]

Bower has recently had issues that have even come up in some of the github issues on this project. More recently though, you will now get this warning installing dependencies on this project:

➜  angular-seed git:(master) yarn install
yarn install v1.0.2
info No lockfile found.                                                              
[1/4] Resolving packages...
warning bower@1.8.2: ...psst! Your project can stop working at any moment because its dependencies can change. Prevent this by migrating to Yarn: https://bower.io/blog/2017/how-to-migrate-away-from-bower/ 

Changes:

• Remove bower in favor of npm/yarn
• Replace all bower_components references with node_modules
• Update the README accordingly
• Add symlink in app/ folder so that <script> tags can load node_modules without the need to copy files or use a build tool -- although a build tool like webpack or brunch should probably be used instead in a follow-up commit because that's what most of this type need and the seed project should set up all the most typical tools for getting the job done right.

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[jaythomas]

I signed it.

[googlebot]

CLAs look good, thanks!

[gkalpak]

This is essentially a duplicate of #390 (and #389). I am going to close it in favor of them, since they address some more issues. Thx taking the time to work on this anyway 👍

@petebacondarwin, should we move forward with #390?