Add additional info for Linux to matrix-seshat building instuctions

Merge remote-tracking branch 'upstream' into develop

Merge remote-tracking branch 'upstream' into develop

Resolve merge conflict: add code from commit 5e882f8 in upstream to s…

…tore.ts

Add Linux support and update to Element 1.12.0 from original repo

Make this shit portable

1. Determine location of user-called Element executable electron file in protocol.ts and, accordingly, the real working directory
2. Use current working directory to store user and session data (electron-main.ts changes), if `--profile` specified, use profile subdirectory for each profile
3. Replace keytar storage with file storage, all user credentials will be stored in encrypted local files (store.ts changes)
4. Create portable build command `yarn build:portable` and clarify build process in README