Improvement to linear regression trend

[Frylen]

This is a small improvement on #918.

Changes:

• Instead of only showing a sign in the trend value when the value is negative, it will now display a + sign on a rising trend and a ± sign on a neutral trend.
• The title of the trend field got updated from "Trend" to "Daily Trend" to show that the trend value is "per day" for the selected timeframe.
• Fixes the calculation, going from 0 to n-1 instead of 0 to n.

[EmilJunker]

@Frylen Please remove the German translation keys and values for "trend" and "trend-line" you added as part of #918 (or alternatively change "Trend" to something like "Täglicher Trend") because otherwise the file will be in conflict with the translation branch from Crowdin when we try to merge it.

[Frylen]

Sorry, I am not sure on how to do that. I do not see any changes to locale-de.json on my end/in the linked pull request.
Do you mean show-total-portion-size in #916?

[EmilJunker]

You are right, my mistake. Somehow my eyes deceived me. #918 indeed didn't contain any changes to the German translations, so this PR can be cleanly merged as it is.