`{` symbol (and any other one) shouldn't break units in non-raw mode

[ta2-1]

Currently if you add { symbol into unit translation it breaks .ftl format. This is wrong because we expect any symbol can be possible.
But there is an issue when you work with plurals:

We don't have a specific UX to input any placeable components. Placeable component can be added as ftl formatted text ({ $num }).
Thus this issue can be solved if we add a specific UX for placeable components. It can be useful just copy them from the source.