Implement context isolation in electron

**Is your feature request related to a problem? Please describe.**

We're not yet following Electron best practices fully. https://www.electronjs.org/docs/latest/tutorial/context-isolation

**Describe the solution you'd like**

https://github.com/bitshares/beet/blob/develop/src/background.js#L170
https://github.com/bitshares/beet/blob/develop/src/background.js#L251

Set `nodeIntegration` and `enableRemoteModule` to false, and `contextIsolation` to true.

Adapt renderer to request node/module functions/data through the ipcmain/ipcrenderer pipeline.

**Describe alternatives you've considered**

Exhaustively prevent XSS in the renderer.

**Additional context**

Latest BTS NFT Viewer has made these changes: https://github.com/BTS-CM/NFT_Viewer/releases/tag/v1.9.2

And the airdrop tool is about 98% of the way there: https://github.com/BTS-CM/airdrop_tool/tree/context_isolation

The background.js file is likely to significantly bloat due to inheriting code from the renderer.

This is likely a multi-thousand LOC feature request, however it's worthwhile to make the wallet follow best practices.