Switched hardcoded subnetwork value to elixir expression for mobile menu.

[YegorSan]

Motivation

Sub-network value for mobile view was hardcoded. I've added Elixir expression in order to get the required value according to selected sub-network.

Changelog

Bug Fixes

Fixed rendering of sub network title in mobile view.

Unfortunately I can not provide screenshot, as locally I can run app only on Sokol Testnet, and if I switch network, it redirects to production app. But I've tested everything, firstly I removed hardcoded value and checked - nothing appeared in menu, then I've added elixir expression, and Sokol Testnet value appeared as it expected.

Checklist for your PR

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so if necessary
• If I added/changed/removed ENV var, I should update the list of env vars in https://github.com/poanetwork/blockscout/blob/master/docs/env-variables.md to reflect changes in the table here https://poanetwork.github.io/blockscout/#/env-variables?id=blockscout-env-variables. I've set master in the Version column.
• If I add new indices into DB, I checked, that they don't redundant with PGHero or other tools