Feature/custom endpoint

[evanpipta]

Addresses Issue: #72

Content:

• Add CustomEndpointsDialogue modal
  • Allows adding custom endpoints
  • Allows removing custom endpoints
  • If the currently active custom endpoint is removed, the mainnet endpoint is selected by default (https://solana-api.projectserum.com)
  • Invalid input rejected with error message (endpoint must start with http:// or https://)
• Add "Manage Custom Endpoints..." to open modal in the network dropdown
• Custom endpoints persisted in local storage
• Add new constant endpoint (https://api.mainnet-beta.solana.com)
• Update network labels

Screen recording:

https://user-images.githubusercontent.com/9023427/106411682-d2e03f80-6413-11eb-8259-8eb798294df6.mp4