Implement network fee slider & custom setting

[rachelhamlin]

Problem

We want to go from this:

To this easy sliding scale, with optional custom setting:

Figma link

Acceptance Criteria

• Any time I tap on Network fee (on the Send transaction screen or the Tx overview), the bottom sheet with sliding scale appears and user can choose a setting.
• If I tap Set custom network fee, I am able to manually enter an amount.

cc @andmironov, designer :)