Set scroll position at top of the textarea when opening the subtitle editor.

## Description

This set the position of the scrollbar at the top of the textarea when opening the __subtitle editor__.
Previously the textarea scroll position was at the bottom of the textarea which doesn't make much sense when you want to edit a subtitle : you most likely want to edit the beginning of the subtitle first.

This also set the caret position on the first character.

## Design decision

I had to use a *component approach* instead of an `<ng-template>` for the edition modal because the `@viewChild` directive doesn't work for elements __inside__ an `<ng-template>`.
I needed the `viewChild` directive to get an `ElementRef` of the `textarea`.

> See the following issue and its workaround :
> - https://github.com/valor-software/ngx-bootstrap/issues/3825
> - https://stackblitz.com/edit/angular-t5dfp7
> - https://medium.com/@izzatnadiri/how-to-pass-data-to-and-receive-from-ng-bootstrap-modals-916f2ad5d66e

## Related issues
Closes [peertube-plugin-transcription/#39](https://gitlab.com/apps_education/peertube/plugin-transcription/-/issues/39)

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