EPIC: `tag` `items` to categorise and organize

As a person who believes that structure is key to organisation and trust in an app
I'd like to be able to tag my inputs with tags I make up myself
So that I can later have a better understanding of what is required from that capture at a glance and filter by the topics I am most interested in.

Tagging and projects are some of the features that I expect to most evolve as part of the app's user experience as they are the most critical but here is a starting point.

To be clear, I don't think that this will be _useable_ as-is, but will certainly be a place to evolve from. We will have a much better UX for this within the next sprint but this is the starting point.

## Acceptance Criteria
+ [x] When I edit an existing `item` in my list of ~~captures~~ `items`,
I see a text field where I can input tags
+ [x] For stage 1, tags are input as a **comma-separated list** (a space required after each comma)
+ [x] A submit button is clicked to save them
+ [x] Each `tag` is checked against the saved tags (case-insensitive) and:
+ [x] If a **`tag` already _exists_**, save the ~~captures~~ `item` against that tag
+ [x] If a **`tag` does _not_ exist**, create a **new `tag`** and save the ~~captures~~ `item` to it
+ [x] Do not worry about leading and trailing spaces for now, this will not be the final UX MVP so we shouldn't waste time on fool-proofing for leading and trailing spaces yet - this version will only be used inside the dwyl team
+ [x] Once `tags` are saved, take human back to the list of ~~captures~~ `items`
+ [x] The list of `tags` should be displayed under the `item.text`
+ [x] each `tag` should be a ***clickable*** `hyperlink`
+ [x] clicking/tapping on the `tag` (`hyperlink`) should filter the `items` in the _current_ `list` to just those with that `tag`.