Adding a new Resource requires an Author, but can't list them all yet

## Problem
When adding a new Resource the Author field is currently required. Currently the list of Authors is restricted to 25 as this is the maximum number of options that can be listed in a Select Menu for discord. [Discord documentation](https://discord.com/developers/docs/interactions/message-components)

We need a new approach for selecting Authors as part of the `/add-resource` command.

I have thought about a few approaches that could be taken, but other ideas are welcome.

### 1) Make Author Optional
This would make it possible to add Resources without an Author and therefore allow submissions to continue.
**Pros**
Simple change
Immediately allows submissions of articles where the Authors are currently not listed
**Cons**
Data quality is reduced
Authors should be searchable

### 2) Add Paging Options
Change the first (apart from the first page) and last options in the Select Menu to be _paging_ selections
**Pros**
Allows all Authors to be listed (eventually)
Possibly a simple code change
**Cons**
Finding an Author near the end of the list will take a long time

### 3) Create an _index_ strategy
Sort all of the Authors into buckets, with a maximum of 25 entries at each level, e.g.

```
A-B
|-> Aa - Ag
|-> Aaron Aardvaark
Aaron Smith
Abbey Mills
...
Agatha Doe
```
**Pros**
Potentially faster average time to find authors
Should be possible to list all authors
**Cons**
Relies on consistent naming of Authors to make sense to users. e.g. Firstname, Surname or Surname, Firstname

### 4) Autocomplete Interaction
It should be possible to create an [Autocomplete Message Interaction](https://discord.com/developers/docs/interactions/application-commands#autocomplete) to allow _searching_ for Authors.
**Pros**
Makes it possible to search for an Author
**Cons**
This does not seem to be supported by discord.js at this time