Increase Token Symbol Length

## What problem are you trying to solve?

**In brief:** The token symbol length requirement of 12 characters is too short, we should raise it to 20 or more if possible.

As expiring tokens become more popular, it is often desired to have the expiration date within the token symbol itself. For example, if I were to create a synthetic yielding token pegged to the USD while backed by ETH and expiring in September of 2020, I might want to use a token symbol of `xyUSDETH-SEP20` (14 characers).

However, some users in our community at UMA have voiced a desire for lexicographically sortable symbol names. And the best way of doing this is to use the ISO standard of [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601) calendar date-inspired format of YYYY-MM-DD. A possible desired symbol might be `xyUSDETH-2020.09.01` (19 characters).

## Describe the solution you'd like

Increase the allowed token symbol length to 20 (or more). The specific length can be further discussed but I think 20 is a reasonable starting point.

## Additional context

Implementation should be quite simple if we follow this previously merged PR: https://github.com/MetaMask/metamask-extension/pull/5816

Review the previously merged MetaMask PR #5816, then search the extension for the current 12-character token symbol limit and its tests. Update the relevant validation and coverage so symbols of at least 20 characters are accepted, and confirm the existing shorter-symbol behavior remains intact.

Written by the indexing model from the issue text.