Improve language in en-US locale strings (#35124)

#35015

For easier review, the changes are split into separate commits by broad
category. The extended commit messages include brief summaries. I am
happy to make a separate pull request for each category if preferred.

While many of the changes are corrections, some are influenced by style.
In those cases I have aimed mainly for consistency throughout the file,
picking a style variant that I think is widely accepted and aids
clarity.

There are additional things that could be improved that I have not
touched. For example, contractions (phrasing variants such as "doesn't"
vs. "does not") could be made more consistent. Not sure how colloquial
or formal the maintainers would like the UI to be.

---------

Signed-off-by: Dominik Rubo <dr-1@posteo.net>
Co-authored-by: Dominik Rubo <dominik.rubo@posteo.net>
Co-authored-by: delvh <dev.lh@web.de>

tests/integration/editor_test.go

@@ -415,7 +415,7 @@ func testForkToEditFile(t *testing.T, session *TestSession, user, owner, repo, b
 		req = NewRequestWithValues(t, "POST", fmt.Sprintf("/%s/%s-1/_edit/%s/%s?from_base_branch=%s", user, repo, branch, filePath, branch), editRequestForm)
 		resp = session.MakeRequest(t, req, http.StatusBadRequest)
 		respJSON := test.ParseJSONError(resp.Body.Bytes())
-		assert.Equal(t, `Branch "master" already exists in your fork, please choose a new branch name.`, respJSON.ErrorMessage)
+		assert.Equal(t, `Branch "master" already exists in your fork. Please choose a new branch name.`, respJSON.ErrorMessage)
 
 		// change a file in the forked repo (should succeed)
 		newBranchName := htmlDoc.GetInputValueByName("new_branch_name")