Rethink case sensitivity comparison for entities

[pavelevap]

History:

• Values for ignored-entities and frequently-written were originally compared as case-sensitive.
• But during work on issue Searching (filtering) commits #586 were values changed to case-insensitive to handle commit filtering in QueryLanguageUtils::createRulesFromQueries, see following commits: 2330858 and b54109f.
• Recognizing capital values for entities was fixed in Capitalized values aren't handled correctly #1429.
• And now we should rethink usage of strtolower() in QueryLanguageUtils::createRulesFromQueries and QueryLanguageUtils::entityMatchesSomeRule.

Plan:

• Move case-insensitive comparison from QueryLanguageUtils::createRulesFromQueries to and QueryLanguageUtils::entityMatchesSomeRule to QueryLanguageUtils::createGitLogQueryFromRule.
• Replace strtolower() with mb_strtolower() - values can be defined in different languages. After moving strtolower() only to keys (for filtering commits) we are safe here, I guess. I created issue Using multibyte string functions #1446 for future general inspection.
• Update documentation: https://github.com/versionpress/versionpress/blob/4475b80e305d3ca46bff7b3770a5e36ef56fc6a5/docs/content/en/developer/plugin-support.md#frequently-written-entities ("Queries use the same syntax as search / filtering in the UI")
• $allowEmpty is not used anywhere:

  versionpress/plugins/versionpress/src/Utils/QueryLanguageUtils.php

  Line 24 in c916d7d

  * @param $allowEmpty boolean Allow empty values

Tests:

• We could add some capitalized values into entityMatchesRightQuery and entityDoesntMatchWrongQuery.
• Revert case-insensitive assertEquals() from queryLanguageUtilsCreatesCorrectRules and queryLanguageUtilsGeneratesCorrectGitLogQuery and check if it works.
• Some tests from QueryLanguageUtilsTest and EntityInfoTest probably duplicates some assumptions now. Yes, they are similar, but EntityInfoTest is testing wrapper functions like isIgnoredEntity() and EntityInfo object, so they are not exact duplicates.