Site redesign discussion

[MaterializeCSS](http://materializecss.com/) is becoming to feel constricting and outdated, and a more frontend-focused design opens up a lot of design space in expanding Notebook.ai moving forward.

Besides a CSS redesign (and potential site restructuring), I'd also like to use a front-end framework like React or Vue.js. If anyone has any thoughts/opinions on either, I'd love to hear them -- I'm very new to both.

**With the redesign, I'd like to accomplish the following larger tasks:**

Back-end improvements to enable a SPA and future mobile apps:
- #55 - Content API

Front-end improvements to help users work in their notebook faster/easier/better:
- #218 - Link to new content (and create it inline)
- #203 - Automatically turn mentions of other content into links
- #7 - Content hovercards
- #60 - Autocomplete on fields with suggested values
- #155 - The tablet UI is very cluttered
- #205 - Content tags & labels

A rewrite of hardcoded content types into user-createable content types
- #206 - Users should be able to create their own content types
- Users should be able to publish their created content types for others to copy/use
- #113 - The ability to toggle content types on/off per universe
- #116 - Content links should be allowed to link to any content type
- #201 - Allowing users to create their own relation fields on content
- #214 - Reorganizing custom (and core) attributes on content types

General polish and improvements to help people customize their notebook to their liking:
- #93 - Site-wide color scheme options
- #216 - Users should be able to select from a list of icons for each piece of content

Per-content-type "apps" that can augment whitelisted content types in various ways:
- #208 - Suggesting character familial relations

**I'll create milestones/issues for each of the above, but basically the big picture changes I'd like to accomplish with the redesign are:**

* A responsive design that looks and works well on phone, tablet, and desktop
* A fast, intuitive UI that makes planning in Notebook.ai faster and more fluid
* Particularly to solve the above frequently-complained-about issues
* More customization available to users in how they want to plan their worlds
* Especially in reusable methods such as letting users design their own content types that can be shared to other users
* Opening up design space to start using big data to provide services to users relating to their content
* For example suggesting familial relationships, pointing out inconsistencies, letting users draw maps, etc

@mpigsley Do you have any thoughts on CSS libraries (or writing our own, and how long/difficult that might be?) or React vs Vue vs something else? Other thoughts welcome (from anyone) also! :+1:

This is a broad redesign discussion rather than an implementation task, and it names no files, tests, or entry points. Review the linked issues and the proposed responsive UI, API, content-type, and customization goals before breaking the redesign into smaller, independently actionable issues.

Written by the indexing model from the issue text.