(core) Improve encoding detection for csv imports, and make encoding an editable option.

Summary:
- Using a sample of data was causing poor detection if the sample were
  cut mid-character. Switch to using line-based detection.
- Add a simple option for changing encoding. No convenient UI is offered
  since config UI is auto-generated, but this at least makes it possible to
  recover from bad guesses.
- Upgrades chardet library for good measure.

- Also fixes python3-building step, to more reliably rebuild Python
  dependencies when requirements3.* files change.

Test Plan:
Added a python-side test case, and a browser test that encodings can
be switched, errors are displayed, and wrong encodings fail recoverably.

Reviewers: alexmojaki

Reviewed By: alexmojaki

Differential Revision: https://phab.getgrist.com/D3979

app/client/components/Importer.ts

@@ -1082,6 +1082,7 @@ export class Importer extends DisposableWithEvents {
               options,
             )
           ]),
+          cssWarningText(dom.text(use => use(this._parseOptions)?.WARNING || ""), testId('warning')),
           dom.domComputed(use => {
             if (use(isSkipTable)) {
               return cssOverlay(t('Skip Table on Import'), testId('preview-overlay'));
@@ -1098,6 +1099,7 @@ export class Importer extends DisposableWithEvents {
         )
       );
     });
+
     const buttons = cssImportButtons(cssImportButtonsLine(
       bigPrimaryButton('Import',
         dom.on('click', () => this._maybeFinishImport(upload)),
@@ -1369,6 +1371,10 @@ export class Importer extends DisposableWithEvents {
         (p: ParseOptions) => {
           anotherScreen.dispose();
           this._parseOptions.set(p);
+          // Drop what we previously matched because we may have different columns.
+          // If user manually matched, then changed import options, they'll have to re-match; when
+          // columns change at all, the alternative has incorrect columns in UI and is more confusing.
+          this._sourceInfoArray.set([]);
           this._reImport(upload).catch((err) => reportError(err));
         },
         () => {
@@ -1517,6 +1523,12 @@ const cssTabsWrapper = styled('div', `
   flex-direction: column;
 `);
 
+const cssWarningText = styled('div', `
+  margin-bottom: 8px;
+  color: ${theme.errorText};
+  white-space: pre-line;
+`);
+
 const cssTableList = styled('div', `
   align-self: flex-start;
   max-width: 100%;

app/client/components/ParseOptions.ts

@@ -1,6 +1,7 @@
 import {bigBasicButton, bigPrimaryButton} from 'app/client/ui2018/buttons';
 import {squareCheckbox} from 'app/client/ui2018/checkbox';
 import {testId, theme} from 'app/client/ui2018/cssVars';
+import {makeLinks} from 'app/client/ui2018/links';
 import {cssModalButtons} from 'app/client/ui2018/modals';
 import {ParseOptionSchema} from 'app/plugin/FileParserAPI';
 import {Computed, dom, DomContents, IDisposableOwner, input, Observable, styled} from 'grainjs';
@@ -60,7 +61,7 @@ export function buildParseOptionsForm(
   return [
     cssParseOptionForm(
       items.map((item) => cssParseOption(
-        cssParseOptionName(item.label),
+        cssParseOptionName(makeLinks(item.label)),
         optionToInput(owner, item.type, optionsMap.get(item.name)!),
         testId('parseopts-opt'),
       )),