Form input does not inherit font-weight

[brnrd87]

If the variable $font-weight-normal is set to a non-default value, e.g. "300", this value is not inherited by text inputs. The computed value of the font-weight property is found to be 'reset' to 400 in the developer tools (Firefox 62.0). When you add 'font-weight: inherit;' to the text input, this is solved. Another solution could be to add a $input-font-weight variable.

[brnrd87]

Reported earlier in #18521, but not as a 'bug', which imho it is, e.g. in case when using custom fonts with both 300 and 400 weights present.

[MartijnCuppens]

The case where font weights of 300 and 400 are available might indeed be a situation where this might come in handy. If we continue on this, we need to make sure we'll also add this for .custom-select.

[andresgalante]

The user agent in chrome will also reset the font weight to 400.

I'll fix this.