Polymer 2 prepare

[balloob]

This is part of #308: the road to Polymer 2.

• No longer depend on "paper-elements" and "iron-elements" but instead depend on explicit packages.
• Updates dependencies to hybrid versions.
• Updates code to use slots instead of attributes.
• Few fixes for bugs that surface when switching to Polymer 2 (bug fixes work on Polymer 1 too)

Note that after this PR, we are still running Polymer 1. However, since it's all hybrid 1 + 2 components, it should work when we switch to Polymer 2. Since there are some things handled differently this is not 100% smooth. After this PR is merged, the cycle will look like: update Polymer 2, find bug, fix bug in Polymer 1 branch. Once no more bugs under Polymer 2 -> switch 👍

[mention-bot]

@balloob, thanks for your PR! By analyzing the history of the files in this pull request, we identified @justweb1, @andrey-git and @turbokongen to be potential reviewers.

[houndci-bot]

Some files could not be reviewed due to errors:

Oops! Something went wrong! :(
ESLint couldn't find the plugin "eslint-plugin-html". This can happen for a couple different reasons:
1. If ESLint is installed globally, then make sure eslint-plugin-html is also installed globally. A globally-installed ESLint cannot find a locally-installed plugin.
2. If ESLint is installed locally, then it's likely that the plugin isn't installed correctly. Try reinstalling by running the following:
    npm i eslint-plugin-html@latest --save-dev
If you still can't figure out the problem, please stop by https://gitter.im/eslint/eslint to chat with the team.

[houndci-bot]

Some files could not be reviewed due to errors:

Oops! Something went wrong! :(
ESLint couldn't find the plugin "eslint-plugin-html". This can happen for a couple different reasons:
1. If ESLint is installed globally, then make sure eslint-plugin-html is also installed globally. A globally-installed ESLint cannot find a locally-installed plugin.
2. If ESLint is installed locally, then it's likely that the plugin isn't installed correctly. Try reinstalling by running the following:
    npm i eslint-plugin-html@latest --save-dev
If you still can't figure out the problem, please stop by https://gitter.im/eslint/eslint to chat with the team.

[balloob]

I have been playing with this, including in compiled mode and all still seems to work fine. Merging this.

Note: this might require some custom UI to be updated to use slots instead of class names.